The Sex Pistols have released a recently-recovered studio demo of the controversial Sid Vicious-penned track "Belsen Was a Gas" that features vocals from frontman Johnny Rotten, Classic Rock Magazine reports.

Recorded in the Sex Pistol's rehearsal room on Denmark Street when Rotten decided to help Vicious with the the lyrics, the vocals are layered with heavy reverb while the instruments are characteristically gritty, if not outright sloppy.

With an overt titular reference to the Bergen-Belsen concentration camp operated by the Nazis, "Belsen Was a Gas" has remained one of the Sex Pistols' more provocative tracks. "The controversial subject matter and dark humor of the song was designed to upset and provoke," said the producers of the box set, "and is certainly an interesting footnote to the Sex Pistols' finite catalogue of songs."